Specification of Reactive Hardware/Software Systems - Electronic ...

5.2 Architecture 119
Requirements Description. An example is a standard architecture (layering) such as
the ISO/OSI model. This model prescribes a (functional) decomposition of a software
system into prescribed layers, and their corresponding interfaces. Such a model enables
the individual redesign or exchange of layers. This can only be done when functionality
and interfaces of the layers is prescribed beforehand.
5.2.3 Modules
The definition of architecture is based on the concept of modularity. Modules are units
that define interfaces. When a system is decomposed (top down) or composed (bottom
up) modules are used to represent the subsystems. Such decomposition into modules
may be physical or logical. Notice that ’the modularity’ of a system is an important
property, and that modularity defines important properties. The design decisions that
cut the system into pieces for example determine their possible reusability.
Some architectures define rather regular structures of (identical) modules. Examples are
transputer networks, systolic arrays, and wave front arrays. Depending on the size of
the modules in relation to the system, there are coarse grain and fine grain architectures.
A coarse grain architecture has relatively few interconnected modules, in contrast to the
many modules in a fine grain architecture.
Especially the specification of distributed systems requires a solid architecture design
before making a more detailed specification. Detailed dynamic behaviour specification
is described in Chapter 6. The style of a behaviour description of a module should be
influenced strongly by its properties. When the modules are physically distributed they
require for example another form of communication than modules that are part of a
monolithic piece of software.
5.2.4 Distribution
A distributed system architecture is often discovered by studying the real world. This is
illustrated in Figure 5.1 and Figure 5.2. These pictures show a physical topology which
is prescribed by geological or spatial properties of the system to be specified. The figures
could for example represent a production line, where Module G is a system controller
and Modules A through D are production machines. The machines may be physically (or
geographically) separated. By identifying an architecture of modules we can prescribe
corresponding boundaries that must be taken into account during analysis. This enables
an early discussion about for example the distribution of intelligence. Module G may be
specified to perform all control tasks, or may be simplified by taking the design decision
to let the Modules A through D perform a part of the control task locally. Such a decision
influences also the possible solution for the interconnect structure. In Figure 5.2 each
connection can use its full bandwidth at any time, whereas the interconnect topology of
Figure 5.1 leads to sharing of a common channel’s bandwidth. Sharing of the channel
leads to behaviour consequences for all modules. For instance the modules might need
an address (identifier) to enable communication.